Virtual machines allow for the running of multiple operating systems on one physical machine. Users of virtual machines may want to save the state of a virtual machine, or to take a snapshot (or multiple snapshots) of a virtual machine in order to preserve a virtual machine state (and perhaps, later in time, to get back to that state). The saving or taking snapshots of virtual machines is usually a time consuming process, since the memory of a typical virtual machine can be large—on the order of several gigabytes or more.
Specifically, preserving the state of virtual devices in virtual machines can be elongated if such virtual devices are busy while a save or a snapshot is taking place. Moreover, the preserving of virtual machine memory is even longer, typically overshadowing the preserving of virtual devices by several orders of magnitude of time. Compounding this problem is the fact that virtual processors are typically stopped while a save or a snapshot of memory is taking place.
Thus, it would be advantageous to provide mechanisms that could preserve the state of a virtual machine and resume the running of a virtual machine while the state-preserving process is still underway. It would also be advantageous to obviate the problem of a virtual machine potentially holding off the execution of a save or snapshot request for long periods of time (or forever) due to virtual devices being unable to preserve their state. Lastly, it would be advantageous to allow for efficient restoring mechanisms that would allow users to restore preserved virtual machine states.